In general, pressurized water nuclear reactors comprise a vessel containing the reactor core, itself supported by an assembly which also forms its screening and constitutes the lower internal equipment of the reactor. Above the core there is arranged a second assembly which forms the upper internal equipment of the reactor and which comprises the guide tubes for the reactor control rods and the upper core plate which is placed in the vessel above the fuel assemblies. The core support assembly which forms the lower internal equipment comprises the core enclosure, in the shape of a cylindrical shell with a vertical axis, and the lower core plate, fixed to the lower part of the enclosure.
The upper core plate is introduced with the upper internal equipment into the vessel, where it takes up a position inside the core enclosure. Between the periphery of this circular horizontal plate and the inner surface of the core there is a predetermined clearance which must be accurately defined and as uniform as possible around the periphery of the upper core plate. This clearance, which is small, and generally barely exceeds 1 mm in the case of a core enclosure whose internal diameter is in the range of 3.40 m, is obtained by providing, when the core enclosure is being constructed, a deposited metal layer, known as buttering, on the inner surface of the core enclosure, in the region where the upper core plate is to be placed. This buttering layer, which projects from the inner surface of the core enclosure, is machined with great care with respect to its diametral accuracy and surface quality. When the upper internal equipment is introduced into the lower internal equipment, the upper core plate may be introduced into the core enclosure without undue difficulty, since the clearance between the general part of the core enclosure and the upper core plate is considerably larger than the clearance in the region of the buttering layer.
In the case where a vessel containing new internal equipment is being outfitted, the operations of adjusting the upper internal equipment and adapting this upper internal equipment to the lower internal equipment and, in particular, checking of the clearance between the peripheral part of the upper core plate and the inner surface of the core enclosure which incorporates the buttering, require numerous checks. However, these checking operations do not present special difficulties, because it is relatively easy to gain access to the upper core plate which is in place inside the lower internal equipment, since this equipment has not been irradiated. These operations are carried out in the factory, during the final stage of manufacture of the internal equipment.
On the other hand, when a nuclear reactor has already been in operation, it may be difficult to carry out the operations of repairing, checking or changing components inside the vessel. It then becomes necessary to operate under water, with the vessel open at its top, full of water and communicating with the reactor pool which may itself also be full of water.
It has recently been proposed to carry out the complete replacement of the upper internal equipment of a pressurized water nuclear reactor, while the irradiated lower internal equipment is kept in the vessel. In fact, it may become necessary to replace the upper internal equipment if it has been subjected to wear or to distortion in use, because of the risk that proper conditions for the guidance of the control rods when the reactor is operating and for their fall in the event of an emergency shutdown can no longer be ensured.
During the operations for adapting new internal equipment to the irradiated internal equipment remaining in the vessel, it is necessary to have available a means for checking, by remote control, the clearance present between the upper core plate and the inner surface of the core enclosure, after the new upper internal equipment has been enplaced inside the lower internal equipment, in the water-filled vessel.